Among the variety of agents for oxidation of organic compounds the most commonly used are derivatives of hexavalent chromium and heptavalent manganese. In terms of their role in elementary kinetic steps, metal ion oxidants usually function as either one or two equivalent reagents. One electron oxidants are those that accept a single electron by direct transfer or by interaction with a hydrogen or halogen atom. Cases have been made for two equivalent oxidants accepting two electrons simultaneously from the substrate; alternatively they accept a hydride ion or lose an oxygen atom. When permangante or chromium (VI) compounds behave as oxidizing agents, the reaction mechanisms are typically very complex. The probable source of selectivity problems with these reagents is that they are in fact best considered to be a family of oxidizing agents rather than a pure reagent. The diversity of chemistry possible in their redox reactions makes for poor selectivity. One very convincing illustration of this point comes from the Cr(VI) oxidation of cyclobutanol. The final inorganic product from the Cr(VI) oxidation of cyclobutanol is Cr(III). Cr(VI) oxidizes cyclobutanol to cyclobutanone. ##STR2## However, an intermediate Cr(IV) leads to ring cleavage to afford the organic product HOCH.sub.2 CH.sub.2 CH.sub.2 CHO. Another substrate susceptible to cleavage during chromic acid oxidation is illustrated below: ##STR3##
An intermediate Cr(IV) complex causes cleavage of the C-C bond to yield PhCHO and (CH.sub.3)COH. The cleavage processes can be suppressed by addition of stoichiometric quantities of manganese(II), catalytic amounts of cerium(II) or cerium(IV), or stoichiometric addition of oxalic acid which intercept the Cr(IV).
Even for ostensibly simple processes where clean reactions occur, complex menchanisms underly the transformations. The oxidation of isopropyl alcohol in 97% acetic acid proceeds by the following sequence of events. ##STR4##
Both Cr(IV) and Cr(V) enter into the isopropanol oxidation process.
Whereas the permanganate oxidation of a primary alcohol to an acid, for instance, will proceed via a series of one or two electron steps, the final metalloproduct is manganese dioxide (or manganese(II) in acid media), corresponding to a three (or five) electron change at the metal. Clearly, intermediate states are involved in the redox process. Permanganate is not a good candidate for oxidizing a secondary alcohol to a ketone, since the reaction is accompanied by carbon-carbon cleavage. Isopropanol oxidation, for instance, leads to formic acid, acetic acid, carbon dioxide, and acetone as reaction products. It seems to be very likely that a major reason for this poor selectivity is that many oxidizing agents are produced in going from Mn(VII) to Mn(IV) or Mn(II).
Clearly one should not describe permanganate or chromium(VI) oxo reagents as single oxidizing agents, but rather each reagent is, in addition to inherently being an oxidizing agent, the precursor to other reagents generated in situ. Rather surprisingly, little attention has been directed to employing manganese(VII) and chromium(VI) as other than "off-the-shelf" reagents, that is to creating new oxido species in order to obtain reactivity and specifity different from that of the parent compounds. There are a large number of studies that have focused on the use of chromium(VI) in different readily available media. Some of the most useful easily prepared oxidizing agents have been discovered in these studies. Each of these reagents exhibits different selectivity and offers different advantages. Examples include Jones' reagent (chromic acid in acetone), Sarett's reagent (CrO.sub.3 in pyridine solvent), Collins' reagent (CrO.sub.3 (py).sub.2 in dichloromethane). Sharpless' reagent (CrO.sub.2 Cl.sub.2, tertiary butanol and pyridine in dichloromethane) and CrO.sub.3 /HMPT.
It should be pointed out that there are a number of selective oxidation reactions involving inorganic oxo-containing reagents-e.g. ##STR5## Both reactions are usually very high yielding. In each case the reaction is also selective at the inorganic component. The clean formal two electron processes Os(VIII).fwdarw.Os(VI) and IO.sub.4.sup.- .fwdarw.IO.sub.3.sup.- are all that occur.